This invention relates to an apparatus and method for in situ monitoring of intraocular pressure.
The human eye contains a flexible transparent structure called the crystalline lens. The function of this structure is to focus light passing through the cornea and pupil onto the photoreceptors which form the retina Although it is termed the crystalline lens, it is composed of living tissue. Cells of the lens are arranged into fibers and it is the precise orientation of groups of these fibers which imparts to the lens its flexibility while maintaining its transparency.
In certain diseases, the arrangement of fibers in the lens becomes disrupted. When this occurs, those regions which are disrupted lose their transparency and become opaque. An opacity is termed a cataract. If a cataract involves a substantial portion of the lens, vision may become impaired. In these cases, it may be necessary to remove the lens surgically and replace it with a lens of synthetic material in order to return sight to the patient.
One of the disease conditions which can lead to cataract formation is an increase in intraocular pressure known as glaucoma. In addition to damaging the lens, an increase in intraocular pressure can damage the neural portions of the eye and lead to permanent blindness. It is possible to treat glaucoma either with drugs or surgery, but to do so successfully, it is necessary to monitor the pressure within the eye.
Typically, an ophthalmologist will measure the pressure within the eye with a tonometer. (See: Textbook of Glaucoma, M. B. Shields, Williams and Wilkins, Pub., 2nd ed, 1987 Chapt. 3, Pg. 54-55) The cornea is topically anesthetized and the tonometer probe is used to flatten cornea. The amount of pressure required to flatten the cornea to a specified amount is a function of the internal pressure in the eye. The difficulty arises that the pressure measurement so obtained is valid only for the instant the measurement was taken. Frequently repeated or continuous measurements are not feasible using this method, and an in situ device is required to monitor intraocular pressure continuously.
Some 20 years ago passive miniature endoradiosondes were developed to measure the Pressure within the eye. (cf. Miniature Passive Pressure Transensor for Implanting in the Eye, C. C. Collins, IEEE Transactions on Bio-medical Engineering, April 1967, p. 74 which is herein incorporated by reference and Intraocular Pressure Transensor Fabrication, C. C. Collins, is herein incorporated by reference) These devices are small enough to be inserted into the eye.
The device is a resonant circuit whose resonance frequency is dependent upon the pressure on the circuit. Therefore, as the pressure within the eye changes, the resonance frequency of the device changes. If the eye and its endoradiosonde are exposed to an electromagnetic field of variable frequency, absorption of energy from the field will occur when the frequency of the field matches the resonance frequency of the endoradiosonde. Therefore, by measuring this frequency, the intraocular pressure of the eye can be determined.
While this technique has been used to measure the intraocular pressure in laboratory animals, it has not been used in humans. The reason for this is that intraocular surgery is a serious procedure which cannot be justified for the implantation of a purely diagnostic device. Further, presence of a free floating sensor in the eye could itself result in further damage to the structures of the eye.
What is required then, is for the sensor to be attached within the eye in such a way that it would not damage the tissues of the eye. Glaucoma patients who require lens removal provide the ideal set of circumstances for the in situ continuous monitoring of intraocular pressure because the device can be placed at or near where the eye's crystalline lens normally would reside.